The present invention relates to a backflow prevention device positioned at an end of a plasticizing and injection screw to prevent backflow of plasticized material into screw flights of the screw.
Injection molding of plastic material involves the use of a screw which is received in a tubular barrel for movement in an axial direction and rotates in the barrel. The screw operates also as a plunger to inject plasticized material into a cavity of an adjacent injection mold, as the screw is moved in axial direction in the barrel. A backflow of plastic material during the injection process is prevented by the use of a backflow prevention device or non-return valve, whereby the backflow prevention device is typically arranged at the forward discharge-side end of the screw. It will be understood by persons skilled in the art that the terms “backflow prevention device” and “non-return valve” are used synonymous in the disclosure.
German Pat. No. DE 198 36 871 describes a non-return valve which includes a screw tip having a tip end, a cylindrical shank connected to the tip end, a thrust ring positioned at a rear end of the shank, and a mounting end extending from the rear end of the shank for attachment to a plasticizing screw. A locking ring is placed in concentric relationship to the central shank and is movable in axial direction between an open position, in which a leading sealing surface rests against a shoulder of the tip end, allowing passage of material, and a closed position, in which a trailing seat surface rests against a shoulder of the thrust ring, prohibiting entry of material. The locking ring together with the shank defines an annular gap and has one end face positioned in opposition to the mounting end and formed with bores for passage of material.
While such a construction of the backflow prevention device enables a rotatable support of the locking ring upon the screw tip in an enclosed bearing, the provision of a kneading or mixing effect by this construction is not contemplated in any way so that material flowing through the backflow prevention device may not be mixed or homogenized sufficiently enough. German Pat. No. 198 36 871 attempts to address this problem of insufficient mixing or homogenization by applying vibration in axial direction in the area of the material passage during rotation of the screw. However, as the applied vibration can only be relatively small, the intended mixing effect is still insufficient.
Another approach to improve a mixing behavior is disclosed in German Pat. No. DE 21 62 709 A1 which describes a non-return valve having a locking ring provided with radially inwardly directed teeth and a screw shank provided with radially outwardly directed teeth. The teeth of the screw shank are disposed in axial offset relationship to the teeth of the locking ring such that the teeth of locking ring and screw shank cannot interlock during movement of the locking ring from the open position to the closing position, or vice versa. To prevent an incidental overlap of the teeth of the locking ring in axial direction with the teeth of the screw shank, the locking ring is restrained in the plasticizing cylinder against rotation.
However, for a number of reasons, the various proposals are endowed with drawbacks and shortcomings relating for example to a complex structure or to the effect that is hoped to be obtained but may not always be realized.
It would therefore be desirable and advantageous to provide an improved backflow prevention device to obviate prior art shortcomings and to realize superior mixing and homogenization results while yet being simple in structure.